The Australian Open, French Open, Wimbledon and US Open have been official majors, recognised by the ITF, since 1924-1925. That is why WCT Dallas and the Masters, as big as they were at the time, were not majors. They were just the tournaments that had the most prize money and were more up to date with the immediate demands of open tennis, unlike the establishment that clung to the old ways as much as possible.